Corrosion behaviour of epoxy coatings investigated by EIS

Epoxy coatings were formed by cathodic electrodeposition of an epoxy resin on steel and on electrochemically modified steel surface by Fe-P alloy using constant voltage method (resin concentration 10 wt.%, temperature 26 degrees C, applied voltage 250 V). The corrosion behaviour of these coatings was investigated during exposure to 3% NaCl using electrochemical impedance spectroscopy (EIS), polarization curves and corrosion potential - time measurements. From the time dependences of pore resistance, coating capacitance, relative permittivity of coating and Warburg diffusion impedance, obtained from EIS, and corrosion current density, obtained from polarization curves, it can be observed that the Fe-P alloy is a good barrier to transport of corrosive agent. During prolonged exposure the Fe-P layer significantly improves stability of protective epoxy system, postponing the time when electrolyte reaches the basic substrate.